Cholesteric texture

More recently it has been shown that it is possible to prepare cholesteric homopolymers.This possibility can lead to strongly twisted cholesterics which may occur in textures reminiscent of those of smectics, especially smectics Such cholesteric substances may yield non-planar fan-shaped, focal-conic or polygonal textures. In the case of homopolymers 

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Rey, A.D. Structural transformations and viscoelastic response of sheared fingerprint cholesteric textures. J. Non-Newtonian Fluid Mech. 1996, 64 (2-3), 207-227. 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

E.P. Raynes, Cholesteric texture and phase change effects, in A.R. Kmetz and... 

Sc Sa, Sa->Ch, and Ch->I respectively. The smectic D phase is observed in the hexyl series but not in the decyl analogs. This observation suggests that the increased flexibility provided by the longer alkyl group destabilizes the higher temperature smectic phases, while stabilizing the smectic C. Optical micrographs of the PDBPB polymer exhibit a fine broken focal conic fan texture for Sq phase, and focal conic fan textures for the Sa phase and cholesteric texture. 

Figure 1. Structure Dickerson dodecamer [9] (left) and prealbumin dimer [10] (right bottom) (Langridge s first examples [11] of electrostatic potential visualizations in combination with skeletal and transparent CPK presentation) as molecular biological structural representatives of the nucleoprotein system, but also as biological representations of individual and relatedness within the frame of structure-phase transitions familiarities between early universe structure computer simulations [12] and cholesteric texture paintings by Lehmann [13,14] (right top).

Fig. 2 POM observations of a concentrated PCIBLA solution (25 wt%) in TCE. The birefringent cholesteric texture changes its sign from (a) left (90°C) to (c) right (102°C) with increasing temperature, (b) The cholesteric pitch diverges in the transition region (97°C). The screw sense of the polypeptide backbone transforms from right to left, accordingly. Reprinted with permission from [76]. Copyright 2005 Wiley-VCH...

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

Fig. 5.8 A photoaddressed and multiswitchable cholesteirc LC display of 6.0 wt% chiral dopant 8 in LC host E7 in a 5 pm thick homeotropic aligned cell (1.5 in. x 1.5 in.). Top schematic cholesteric textures Middle demonstration of an image Bottom crossed polarized textures. Reproduced with permission from [76]. a Planar state, b Focal conic state, c Planar state. Copyright 2011 John Wiley Sons...

As stated in the introduction, the first LC thermosets were prepared in 1973 by Blumstein and Strzelecki et al. [6,53-56] via the thermal polyaddition of acrylate-terminated schiff-base mesogens within the mesophase. Strzelecki et al. also reported the first preparation of networks with a helicoidal structure. Copolymerization of the mixture of LC diacrylate, LC monoacrylates having a mesogen, and/or a chiral group within the cholesteric mesophase resulted in the LC networks with a characteristic cholesteric texture. 

Fig. 4.29 Measurements of the pitch of the helix in a cholesteric, (a) Geometry for mraiochro-matic light diffraction on the focal-conic texture. The pitch is found from the angle 2i between the incident and diffracted beams with wavevectors ko and kr, (b) Spectral measniemtaits of the light transmissicm by a planar cholesteric texture (Jo and 7t are intensities of the incidoit and transmitted beam)...

Fig. 12.1 Spectra of optical rotatory power (OR) and selective reflection (R) of a planar cholesteric texture for light propagation parallel to the helical axis.

Fig. 12.8 Influence of a magnetic field H on the planar cholesteric texture having Xa > 0. The helical axis is parallel to z. Horizontal lines show the projections of the director parallel to H. Helix unwinding according to de Gennes (a) and Meyer experiment (b)...

The calculated field induced transmission of the same planar cholesteric texture in the non-polarized light is shown in Fig. 12.12. It is clearly seen that, with increasing field, the Bragg minimum is only slightly shifted to shorter wavelengths due to a distortion of the helix seen in the previous figure and then disappeared at a field of about 25 V/pm. Therefore, in the absence of defects the field cannot increase the period of the helix. An essential increase of the cell thickness does not influence the result. The measurements of the field dependence of the transmission spectra of a cholesteric with the same parameters have confirmed the absence of the red shift of the Bragg minimum [15]. 

Fig. 12.12 Calculated optical transmission spectra of the planar cholesteric texture as functions of the electric voltage applied (unpolarized light). Principal refraction indices used are ri = 1.550, = 1.474, for other parameters see Pig. 12.11...

Fig. 12.14 Calculated transmission spectra of a planar cholesteric texture in zero field and in field = 5.7 V/)im applied perpendicular to the helical axis. Note appearance of the strong second order photonic stop-band even for nonpolarized light...

The cholesteric texture occurring under the conditions of parallel boundary orientation is a featureless, twisted planar... 

The cholesteric texture occurring with perpendicular boundary orientation, on the other hand, is not a uniform texture. Under the microscope the entire field of view is filled with right-and left-handed spirals. This cholesteric texture is known as the scroll texture and appears very much like an end-on view of a bundle of rolled-up scrolls. Capacitance measurements show that the cholesteric helical axis in this texture is still predominantly perpendicular to the plane of the layer. An exact analysis of the structure of this texture, even for the case of equal elastic constants, seems difficult to achieve. Important from the applications point of view is that the scroll texture is adopted without disclinations immediately after a display element is turned off and that this structure is essentially nonscattering. Homeotropic boundary orientation therefore solves the problem of display after-images. 

Polymer-dispersed LC (PDLC) [30], polymer-stabilized cholesteric texture (PSCT) [31], and LC gels [32] all exhibit optical scattering characteristics and have wide applications in displays and optical devices. The LC gel-based reflective LCD can also be extended to transflective... 

J. W. Doane, D.-K. Yang, and Z. Yaniv, Front-lit flat panel display from polymer stabilized cholesteric textures, Proc. Japan Display 92, 73 (1992). 

Figure 11.2 (a) Photo of the light shutter from polymer stabilized cholesteric texture (PSCT) normal... 

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